1. Field of the Invention
The present invention relates to a dynamic, class-A FET amplifier, and more particularly, to a class-A FET amplifier which includes a signal envelope detector which is used for dynamically controlling the gate supply voltage in accordance with the envelope of the input signal.
2. Description of the Prior Art
There has been an on-going effort to increase the linearity, dynamic range, stability, and power conversion efficiency of power amplifiers. As more of these devices are used at higher frequencies, notably microwave frequencies, there is a need to improve the efficiency of these devices.
One amplifier circuit with an improved degree of efficiency is disclosed in U.S. Pat. No. 3,961,280 issued to T. Sampei on June 1, 1976. The Sampei circuit comprises a plurality of separate power sources for generating different voltages, and means for changing, in steps, the power supply voltages applied to the collector (or drain) input of a transistor amplifying element as a function of the instantaneous amplitude of the input signal. The change in collector power supply thereby allows the amplifying element to provide approximately the maximum output signal for a varying amplitude input signal. Such an arrangement, however, becomes large and cumbersome to operate at higher frequencies, and following the instantaneous amplitude of a microwave frequency input signal is extremely difficult.
An alternative arrangement is disclosed in U.S. Pat. No. 3,984,783 issued to R. H. Bickley on Oct. 5, 1976, which relates to an amplifying circuit capable of providing the gain and linearity of a class A amplifier, while also providing the power conversion efficiency of a class B or C amplifier. As disclosed, the Bickley device includes a biasing network and switch, where the switch is responsive to the amplitude of the input signal and functions to alter the bias network so as to allow the amplifier to function alternatively in class A, B, or C mode, depending on the amplitude of the input signal. However, as with the previously described arrangement, the Bickley arrangement becomes impractical at microwave frequencies, where the amplitude of the input signal changes quickly as a function of time.
A problem remaining in the prior art, therefore, is to provide an arrangement for increasing the power conversion efficiency of power amplifiers which operate at microwave frequencies.